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AD8018 Datasheet(PDF) 8 Page - Analog Devices |
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AD8018 Datasheet(HTML) 8 Page - Analog Devices |
8 / 19 page REV. A AD8018 –8– THEORY OF OPERATION The AD8018 is composed of two current feedback amplifiers capable of delivering 400 mA of output current while swinging to within 0.5 V of either power supply, and maintaining low distortion. A differential line driver using the AD8018 can provide CPE performance on a single 5 V supply. This performance is enabled by Analog Device’s XFCB process and a novel, two- stage current feedback architecture featuring a patent-pending rail-to-rail output stage. A simplified schematic is shown in Figure 4. Emitter followers buffer the positive input, VP, to provide low input current and current noise. The low impedance current feedback summing junction is at the negative input, VN. The output stage is another high-gain amplifier used as an integrator to provide frequency compensation. The complementary common-emitter output provides the extended output swing. A current feedback amplifier’s dynamic and distortion performance is relatively insensitive to its closed-loop signal gain, which is a distinct advantage over a voltage-feedback architecture. Figure 5 shows a simplified model of a current feedback amplifier. The feedback signal is a current into the inverting node. RIN is inversely proportional to the transconductance of the amplifier’s input stage, gmi. Circuit analysis of the pictured follower with gain yields: VV G T TR G R OUT IN ZS ZS F IN / () () =× ++ × where: GR R T R CR Rg FG ZS T S TT IN mi =+ = + =≅ 1 1 1 125 / () / () Ω Recognizing that G RIN < RF, and that the –3 dB point is set when TZ(S) = RF, one can see that the amplifier’s bandwidth depends primarily on the feedback resistor. There is a value of RF below which the amplifier will be unstable, as an actual ampli- fier will have additional poles that will contribute excess phase shift. The optimum value for RF depends on the gain and the amount of peaking tolerable in the application. VO BIAS VN VP Figure 4. Simplified Schematic G = 1 IT = IIN CT RT IIN VOUT RG RF RIN + VIN – VO + – Figure 5. Model of Current Feedback Amplifier FEEDBACK RESISTOR SELECTION In current feedback amplifiers, selection of the feedback and gain resistors will impact the MTPR performance, bandwidth, noise, and gain flatness. Care should be exercised in the selection of these resistors so that the optimum performance is achieved. Table I shows the recommended resistor values for use in a variety of gain settings for the test circuit in TPC 1. These values are intended to be a starting point when designing for any application. FREQUENCY – Hz –90 1M 10M 100M 1G –80 –70 –60 –50 –40 –30 –20 –10 RL = 5 SIDE A DRIVEN RL = 5 SIDE B DRIVEN VIN = 2V p-p G = 2 VS = 2.5 100k RL = 100 SIDE A DRIVEN RL = 100 SIDE B DRIVEN –100 –110 TPC 25. Crosstalk vs. Frequency |
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